What puzzles me is that the Bristol route has proven its ability to attract 45000 passengers per annum and yet seemingly no other airline is interested in the route following its dropping by Ryanair.
I think you're only looking at half the picture. Surely what has been demonstrated is that Bristol can attract 45,000 people prepared to pay Ryanair yields? Another carrier thinking about entering this market with more appropriate capacity (i.e. smaller aircraft, so higher per-seat cost) would be unlikely to be able to make money on typical Ryanair ticket prices, but higher prices would mean fewer passengers. (Is there another point on the supply-demand curve that would also work? 20,000 passengers at significantly higher fares? Perhaps, but gambling on new routes in the current market is risky business.)